1. Field of the Invention
Embodiments of the invention relate generally to the field of antenna systems and more specifically to receiving antennas for satellite-based positioning systems.
2. Description of the Related Art
Conventional satellite-based positioning systems, for example, a Global Navigation Satellite System (GNSS) such as the Global Positioning System (GPS) include a GPS receiver system. An important part of the receiving system is the antenna. GNSS satellites typically broadcast at two frequencies, 1.575 GHz, which is referred to as the L1 signal, and 1.225 GHz, which is referred to as the L2 signal. Therefore GNSS antennas may have to be capable of receiving signals at both frequencies.
Non-ideal behavior of the antenna presents limitations in determining position with very high accuracy. Optimally, the antenna would receive only direct signals from the satellite with very high electrical phase stability, regardless of the elevation and azimuth angles of the satellite. The antenna should have a means for rejecting signals that have become corrupted by reflection, diffraction and/or refraction from physical structures in the vicinity of the path (or paths) of the signals arriving at the receiving antenna. The satellites transmit towards the earth with Right Hand Circular Polarization (RHCP). The best simple receiving antenna, used by a conventional GPS receiving system, will be responsive only to RHCP signals. The response of the antenna to Left Hand Circular Polarization (LHCP) should be many decibels down from that of the RHCP over a wide angular range. This type of antenna will be referred to as a High Purity Circularly Polarized (HPCP) antenna. A good high RHCP over LHCP response corresponds to a low axial ratio, which is the magnitude of the RHCP plus the magnitude of the LHCP all divided by the magnitude of the RHCP minus the magnitude of the LHCP for a given angular position in space when the antenna is exposed to a pure Linearly Polarized EM wave. An RHCP antenna should have a high ratio of RHCP over LHCP, which corresponds to a low axial ratio. A 20 dB RHCP to LHCP ratio corresponds to an axial ratio of 1.75 dB and a 24.8 dB RHCP to LHCP ratio corresponds to an axial ratio of 1.00 dB.
Many types of circularly polarized (CP) antennas are available for consideration. Some of the widely used CP antenna types include the CP microstrip patch, helical, spiral slot radiator, crossed electric dipoles (or turnstile), crossed slots, conical spiral antennas among others. The various antennas discussed above all have various shortcomings for achieving the desired high performance GPS antenna with two outputs, RHCP and LHCP. Microstrip patch antennas are likely to be too narrow band. Helical and spiral antennas can be built for RHCP or LHCP but not for both outputs simultaneously. The turnstile antenna can be built to deal with both of the above problems but it has a very poor axial ratio in the plane of the dipoles. In fact, it is difficult to obtain a good axial ratio over a wide angular range (over the upper hemisphere) with virtually any circularly polarized antenna.
In high accuracy applications the mathematical processes utilized in the GPS receiver and subsequent digital processors determine the number of wavelengths and the number of electrical degrees between that satellite and the GPS receiving antenna phase center. It is therefore important that the GPS antenna have a phase center that stays in the same location within very small tolerances as the reception angle of a given incoming wave changes from near the horizon to the zenith. The phase center should also be independent of azimuth reception angle and be fairly independent of the frequency in use.
The use of crossed dipoles for generation of circular polarization is well known. The dipoles have a common center point, lie in one plane at right angles to each other and are fed by signals that are 90 degrees out of phase. This structure with feed lines and mounted above a ground plane, without the circular waveguide, has been called the “turnstile antenna.” See Sichak, W. and S. Milazzo, “Antennas for Circular Polarization” Proc. IRE, Vol. 36, No. 8, August 1948, pp. 997-1001; Wilkinson, W., O. Woodward and W. Mulqueen, “Two Communication Antennas for the Viking Lander Spacecraft”, IEEE APS Int. Sym., June 1974, Vol. 12, pp. 214-216; and U.S. Pat. No. 4,062,019 to Woodward et al. The placement of the turnstile antenna inside or near a circular waveguide cavity has been carried out. See U.S. Pat. No. 3,740,754 to Epis, U.S. Pat. No. 3,789,416 to Kuecken, and U.S. Pat. No. 4,109,256 to Woloszczuk. These authors have referred to the circular waveguide, with the top end open and the bottom end shorted out, as a “cup” and as a “cavity.” More recently reports have appeared showing the use of a cup with interior patch antennas. See Gao, S. et al., “Antennas for modern small satellites,” IEEE Antennas and Propagation Magazine, Vol. 51, No. 4, pp. 40-56, August 2009. The two dipoles must be fed signals that are phase shifted by 90 degree electrical relative to each other. For narrowband antennas the phase shift may be obtained by detuning the two dipoles, one tuned to a higher frequency and the other tuned to a lower frequency. For broadband applications it is necessary to use a device such as a branch line coupler or a quadrature hybrid coupler. See Rao, K. S., J. Kopal, M. Q. Tang, and S. G. Gupta, “A High Performance Circularly Polarized Feed Array for Satellite Communication Antennas,” IEEE APS Int. Sym. June 1989, Vol 3, pp. 1420-1423. Some have used printed circuit board techniques for building a “Crossed-Drooping Dipole Antenna” for circular polarization. See U.S. Pat. No. 4,686,536 to Allcock.
The turnstile and circular waveguide cavity circularly polarized antenna can be built to produce high purity right hand circularly polarized radiation in the upward direction. However, there is strong tendency for most RHCP antennas radiating upward to radiate LHCP in the downward direction. This is undesirable as the antenna can receive reflected signals from the ground. These signals would originally be RHCP but on reflection from the ground they will become LHCP which can enter the antenna from the backward direction. It is therefore desirable to build the antenna to suppress reception of LHCP signals coming from the backward or downward direction. A dipole placed above a circular ground plane with a diameter of about 0.5 to 1.0 wavelengths will give a front to back ratio of about 8 to 14 dB. See Tranquilla, J. M. and R G. Colpitts, “Development of a class of antenna for spacebased Naystar GPS applications”, 6th Int. Conf. Ant. & Prop., ICAP 89, April 1989, Vol. I, pp. 65-69; Scire-Scappuzzo, F., and S. N. Makarov, “A Low-Multipath Wideband GPS Antenna with Cutoff or Non-Cutoff Corrugated Ground Plane”, IEEE Trans. A&P, Vol. 57, No. 1, pp. 33-46, January 2009. For GNSS applications, a device known as a “choke ring” is widely used to isolate the basic RHCP antenna from LHCP signals coming from the direction of the ground. Choke rings are generally large with a diameter of 36 cm or more, with weights of a few Kg or more and are expensive, but they reduce the back LHCP radiation by about 10 to 15 dB.
Referring to FIG. 1, the antenna described in U.S. Patent Publication 20090204372 (which is hereby incorporated in its entirety by reference) uses two sets of dipoles with each set having two dipoles tuned to two frequencies. These dipoles may act like tuned circuits that are closely coupled to each other and may be regarded as coupled resonant circuits. Traditionally, it is known that over-coupled resonant circuits give a poor match and poor transmission of power at intermediate frequencies. See “Electronic and Radio Engineering,” T. E. Terman, McGraw-Hill Inc. 1955, Fourth Edition, pp. 63-72, Sec. 3.5, entitled “Behavior of Systems Involving Resonant Primary and Secondary Circuits.” FIG. 2 shows a basic antenna also found in the prior art having a circular waveguide cavity with the top end open and the bottom end closed, dual crossed dipoles, no back radiation suppression disk, and a mounting stem.